Recently, microwave dielectric ceramics have received increasing attention because of their applications in microwave resonators and filters where their high dielectric constant and high quality factors are required. To meet the requirement of small, thin, light mobile wireless communication systems, multilayer ceramic devices cofired with low loss conductors such as silver and copper are desired. This is also the major purpose of the present invention disclosure.
There are various microwave ceramic materials such as TiO.sub.2, Ba2Ti.sub.9 O.sub.20, BaTi4O.sub.9, ZrO.sub.2 --SnO.sub.2 --TiO.sub.2, Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 and (Ba,Pb)Nd.sub.2 Ti.sub.5 O.sub.4. However, all these materials have sintering temperatures above 1300.degree. C. and thus cannot be cofired with highly conductive metals, such as Ag and Cu. Several approaches can be taken to reduce the densification temperature of the above ceramic dielectric systems below 1000.degree. C. or less, such as by using fine powders, adding sintering flux or low-softening glass. Adding a low-softening borosilicate glass to pure TiO.sub.2 was chosen in the present invention.
U.S. Pat. No. 5,449,652 discloses a microwave dielectric ceramic composition comprising Bi.sub.(2-x) (Zn.sub.(2+y)/3 Nb.sub.(4/3))O.sub.(7-3x/2+y/3) wherein 0.24&lt;x&lt;0.333, 0.120&lt;y&lt;0.3; and Bi.sub.(1-z) Ca.sub.(z) (Zn.sub.(2+y)/3 Nb.sub.(4/3))O.sub.(7-3x/2+y/3+xz/-z) wherein 0&lt;x&lt;0.667, 0&lt;y&lt;0.30, 0&lt;z&lt;0.2. The microwave dielectric ceramic composition have properties including a dielectric constant(@ 7 GHz) of 100, quality factor (@ 7 GHz) of 7000 and a temperature coefficient of 10 ppm/.degree. C.
U.S. Pat. No. 4,674,152 describes a ceramic composition with a low dielectric constant which comprises 50-95 wt % crystallizable glass and 5-50 wt % ceramic filler. The dielectric system has a dielectric constant of 5.1-6.0. The crystallizable glass consists of 5-20 wt % lithium oxide, 60-90 wt % silicon dioxide, 1-10 wt % aluminum oxide, and 1-5 wt % alkaline metal oxide other than lithium oxide. The ceramic fillers include silicon dioxide and aluminum oxide.
U.S. Pat. No. 4,755,490 describes a ceramic composition with a low dielectric constant which comprises 10-50 wt % alumina, 0-30 wt % fused silica, and 50-60 wt % of a frit comprised of 4 wt % CaO, 12 wt % MgO, 29 wt % B.sub.2 O.sub.3 and 42 wt % SiO.sub.2. The dielectric composition has a sintering temperature below 1000.degree. C., a dielectric constant of 4.5-6.1, and a linear thermal expansion coefficient of 3.9-4.2.times.10.sup.-6 K.sup.-1.
U.S. Pat. No. 5,415,945 describes a ceramic composition with a high dielectric constant which comprises 75-85 mol % Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3 +0-15 mol % PbTiO.sub.3 +5-16.5 mol % Pb(Zn.sub.1/2 W.sub.1/2)O.sub.3 +Pb(Cu.sub.1/3 Nb.sub.2/3)O.sub.3. The composition has a sintering temperature of 1000.degree. C. and a dielectric constant of 1000-4000.
U.S. Pat. No. 5,262,368 describes a ceramic composition with a high dielectric constant which comprises BaTiO.sub.3, BaCuO.sub.2, WO.sub.3 and MoO.sub.3. The system has a sintering temperature of 1150.degree. C., a dielectric constant of 2000-3000 (1 KHz) and a dielectric loss of 2.5%-16% (1 KHz).
U.S. Pat. No. 5,461,014 describes a ceramic composition with a high dielectric constant which comprises Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 and BaCuO.sub.2. The system has a sintering temperature of 1050.degree. C., a dielectric constant of 7000-8000 (1 KHz) and a dielectric loss less than 3%.
Taiwan Patent Publication No. 159830 describes a ceramic composition with a high dielectric ceramic composition which is comprises Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 --PbTiO.sub.3 --Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3. The system has a sintering temperature of less than 1000.degree. C. and a dielectric constant of 8000-10000.
In view of the above development, it is obvious that ceramic materials which have a low sintering temperature, a high dielectric constant and a high quality factor (i.e. low loss) are needed in the industry. The present invention discloses ceramic compositions which meet the above requirements.